Electromagnetic wave absorber, method of manufacturing the same and appliance using the same

ABSTRACT

An electromagnetic wave absorber for use in the high frequency range above 1 Ghz and a composite member are characterized by the fact that magnetic metal grains are covered with ceramic above 20 volume %. Further, a method of manufacturing the electromagnetic absorber and the composite member is characterized by the fact that composite magnetic particles, in which a plurality of magnetic metal grains and ceramic are unified, are formed through a mechanical alloying method applied to a composite powder composed of magnetic metal powder and ceramic powder. The electromagnetic wave absorber can be used in a semiconductor device, an optical sending module, an optical receiving module, an optical sending and receiving module, an automatic tollgate in which erroneous operation due to electromagnetic wave disturbance is provided by use of the electromagnetic wave absorber.

BACKGROUND OF THE INVENTION

The present invention relates to a novel electromagnetic wave absorber,a method of manufacturing the electromagnetic wave absorber, a compositemember and an appliance of the electromagnetic wave absorber. Moreparticularly, the invention relates to an electromagnetic wave absorbercomprising composite magnetic particles composed of magnetic metalgrains and ceramics, particularly, fine crystalline grains containing atleast one kind of material selected from the group consisting ofnon-magnetic or soft magnetic metal oxides, carbides and nitrides. Theinvention also relates to a method of manufacturing the electromagneticwave absorber, a composite member using the electromagnetic waveabsorber, and a semiconductor device, a printed wire board, an opticalsending and receiving module, an electronic toll collection system andan electronic device casing using the magnetic metal particles.

In accordance with the present invention, an optical sending module, anoptical receiving module or an optical sending and receiving moduleintegrating the optical sending module and the optical receiving modulefor use in a high speed communication network using optical fibers canbe obtained, and the modules can be made small in size, light in weight,high in processing speed and high in sensitivity by suppressing noisesemitted to the outside and noise interference inside the module.

In recent years, there have been significant advances in high speedprocessing in electronic equipment, and the operating frequency of anIC, such as an LSI or a microprocessor, has rapidly increased, with theresult that undesirable noises are likely to be emitted.

Further, in the field of communication, the GHz band electromagneticwaves are proposed for use in the next generation of multimedia mobilecommunication (2 GHz) and in wireless LANs (2 to 30 GHz). In the fieldof the Intelligent Transport System (ITS), the Electronic TollCollection System (ETC) uses 5.8 GHz electromagnetic waves, and theAdvanced Cruise-assist Highway System (AHS) uses 76 GHz electromagneticwaves. It is expected that the range of use of high frequencyelectromagnetic waves will rapidly expand even further in the future.

As the frequency of electromagnetic waves is increased, anelectromagnetic wave is apt to be emitted as a noise. Thus, in recentelectronic equipment, due to a decrease in the noise margin due to areduction in the electric power consumed by the equipment and by adecrease in the immunity (noise resistance) due to replacement ofdigital circuits to analogue circuits and the tendency of small-sizingand high-mounting density, the noise environment inside the equipmenthas been deteriorated, thereby to cause a problem of erroneous operationof the equipment due to electromagnetic interference (hereinafter,referred to as EMI).

Therefore, measures have been taken to reduce the EMI inside theelectronic equipment by placing an electromagnetic wave absorber in theelectronic equipment. As an electromagnetic wave absorber for the GHzband, a sheet composed of an electrically insulating organic material,such as rubber, a resin or the like, and a magnetic lossy material, suchas a soft magnetic metal oxide, a soft magnetic metallic material or thelike, is mainly used.

However, the electric resistivity is around 500 to 1000 μΩ·, which isnot so high. Therefore, a decrease of the magnetic permeability due toeddy currents in the GHz region is inevitable. Further, in regard to thecomplex specific dielectric constant, since the imaginary part is largecompared to the real part, because the electric resistivity is notsufficiently high, it is difficult to adjust the impedance matching.

In general, characteristics required for the electromagnetic waveabsorber for electronic information-and-communication equipment are{circle around (1)} a large reflection attenuation coefficient (smallreflection coefficient), {circle around (2)} a wide band capable ofabsorbing electromagnetic waves, and {circle around (3)} a smallthickness. However, no electromagnetic wave absorber capable ofsatisfying all of these characteristics has been developed as yet.

In order to attain the above item {circle around (1)}, it is necessarythat the amount of electromagnetic waves reflected on the surface of theabsorber is small. In order to do so, it is necessary to make the value√{square root over ( )}(μ_(r)/∈_(r)) of the characteristic impedance ofthe substance close to the value √{square root over ( )}(μ₀/∈₀) of thecharacteristic impedance of the free space. Therein, μ_(r) is a complexspecific magnetic permeability μ_(r)(μ_(r)′+jμ_(r)″), ∈_(r) is a complexspecific dielectric constant ∈_(r)(∈_(r)′+j∈_(r)″), and μ₀ and ∈₀ arethe magnetic permeability and the dielectric constant of the free space,respectively. In order to attain the above item {circle around (2)}, itis necessary that the values μ_(r)′ and μ_(r)″ are graduallymonotonously decreased with respect to frequency, while the relationshipbetween the values μ_(r)′ and μ_(r)″ is being kept nearly constant. Inorder to attain the above item {circle around (3)}, it is necessary thatthe amount of attenuation of electromagnetic waves inside the substanceis made large. In order to do so, it is necessary that the real part ofthe propagation constant (γ=2πf(μ_(r), ∈_(r))^(0.5)) of the substance islarge, that is, the values of the complex specific magnetic permeabilityand the complex specific dielectric constant at a desired frequency aremade large. However, as the value of the complex specific magneticpermeability becomes large, it is difficult to adjust the impedancematching of the substance with the free space.

Since the soft magnetic metal oxide material of spinel crystal structureas a proven electromagnetic absorber has an electric resistivityextremely higher than that of the soft magnetic metallic material, themagnetic permeability rapidly decreases in the GHz band though thereflection by eddy current is small. Therefore, a considerably largethickness is required in order to well absorb the electromagnetic waves.

On the other hand, the soft magnetic metallic material offers thepossibility of providing a thin electromagnetic wave absorber, becausethe specific magnetic permeability is very high. However, in the highfrequency region, the specific magnetic permeability is substantiallydecreased and the imaginary part of the complex specific dielectricconstant is substantially increased due to eddy current loss because theelectric resistivity is low. Therefore, the reflection becomes large,and the soft magnetic metallic material does not work as anelectromagnetic wave absorber.

In order to solve the problem described above, Japanese PatentApplication Laid-Open No.9-181476 proposes to use an ultra-finecrystalline magnetic film of a hetero-granular structure in whichferromagnetic ultra-fine crystalline metallic phases are dispersed in ametal oxide phase as an electromagnetic wave absorber in a highfrequency range. Such a magnetic film is characterized in that softmagnetism is provided by the ferromagnetic ultra-fine crystals and highelectrical resistivity is provided by the metal oxide phase, and therebythe eddy-current loss is reduced and a high magnetic permeability in thehigh frequency range can be obtained.

The method of manufacturing the electromagnetic wave absorber is thatthe soft magnetic metal and oxygen, nitrogen, carbon are sputteredtogether with a metal oxide phase constitutive element having anaffinity with the above elements at a time to form an amorphous filmcontaining these elements on a substrate such as an organic film, andthen the film is heat-treated to form a two-phase structure by producingthe ferromagnetic ultra-fine crystals in the metal oxide phase. However,the electromagnetic wave absorber has problems in that the cost is highbecause a large film-forming apparatus is required, and the use of theelectromagnetic wave absorber is limited because of the thin-filmstructure.

Japanese Patent Application Laid-Open No.7-212079 and Japanese PatentApplication Laid-Open No.11-354973 disclose an electromagnetic waveinterference suppresser or an electromagnetic wave absorber composed ofoblate shaped soft magnetic metal particles and organic bond. The softmagnetic metal particles are formed in an oblate shape having athickness thinner than the skin depth to suppress eddy current, and animprovement in the magnetic resonance frequency is achieved by theeffect of shape magnetic anisotropy, and improvement of magneticpermeability is achieved by reducing of the demagnetization field causedby the shape. As the result, an excellent electromagnetic waveabsorption ability is obtained in the range of several MHz to 1 GHz.However, it does not have a sufficient thickness and absorption abilityas an electromagnetic wave absorber used inside of electronic equipmentor used for a high frequency region.

Further, Japanese Patent Application Laid-Open No.9-111421 proposes amagnetic material for loading coils which obtains high electricresistivity in a high frequency region by heat-treating a highmagnetic-permeability amorphous alloy at a temperature above thecrystallization temperature in an atmosphere containing at least onekind of gas selected from the group consisting of oxygen gas, nitrogengas and ammonia gas to form crystal grains made of a high magneticpermeability alloy and oxide or nitride around the crystal grains.

Furthermore, Japanese Patent Application Laid-Open No.11-16727 proposesa magnetic thin film for high frequency magnetic elements composed ofiron having ferromagnetism and nickel ferrite having magnetism, andhaving a structure of dispersing a magnetic phase in a ferromagneticphase or the ferromagnetic phase in the magnetic phase, or laminatingthe ferromagnetic phase and the magnetic phase in a multilayerarrangement. However, this publication does not propose to use themagnetic thin films as an electromagnetic wave absorber.

Further, Japanese Patent Application Laid-Open No.9-74298 proposes anelectromagnetic wave shield material formed by mixing ceramic andmagnetic grains in a ball mill using a silicon nitride ball, and thensintering the mixture. However, the publication does not propose anyelectromagnetic wave absorber.

Further, in regard to the optical sending and receiving module, apreventive measure of internal interference caused by sending andreceiving noises between the optical sending part and the receiving partis disclosed in Japanese Patent Application Laid-Open No. 11-196055.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a thin electromagneticwave absorber which has excellent electromagnetic wave absorbingcharacteristics in the high frequency range and which can bemanufactured with a smaller number of production processes, and toprovide a method of manufacturing the electromagnetic wave absorber, acomposite using the electromagnetic wave absorber, and an applianceusing the electromagnetic wave absorber.

Another object of the present invention is to provide an optical sendingmodule, an optical receiving module and an optical sending and receivingmodule which can be made small in size, light in weight, high inprocessing speed and high in sensitivity using an electromagnetic waveabsorber which has good applicability, and has electromagnetic waveabsorption characteristics which are not deteriorated even at atransmission speed above 2.4 GHz.

An electromagnetic wave absorber in accordance with the presentinvention is characterized by the fact that it comprises compositemagnetic particles preferably having a grain size smaller than 10 μm,particularly smaller than 5 μm, in which magnetic metal grains andceramic are unified, preferably, magnetic metal grains and ceramic above10%, preferably above 20%, in volume ratio are unified; and by the factthat it comprises composite magnetic particles in which a plurality offine magnetic metal grains and ceramic are unified by enclosing theplurality of fine magnetic metal grains with a ceramic; and by the factthat it comprises composite magnetic particles in which a plurality ofmagnetic metal grains and ceramic are unified by embedding ceramics,preferably having a bar-shape, into magnetic metal grains.

That is, an electromagnetic wave absorber in accordance with the presentinvention is characterized by the fact that it comprises compositemagnetic particles in which a large number of fine magnetic metalgrains, preferably smaller than 0.1 μm, particularly smaller than 50 nm,and ceramic above 10 volume %, preferably 20 to 70 volume %, areunified. Particularly, the magnetic metal and the ceramic are formed inalternatively laminated layers in each grain, and the magnetic metal isprovided in the form of complicated shaped particles, and the size ofmost of the particles is smaller than 100 nm, and the particles areenclosed with a ceramic. The complicated shaped particle is formed bygathering fine particles having a particle size smaller than 20 nm. Mostof the ceramic is formed in a shape surrounding the magnetic particles,and a small amount of the ceramic is formed as bar-shaped grains.

It is preferable that the magnetic metal is at least one kind of metalor alloy selected from the group consisting of iron, cobalt and nickel,and the ceramic is at least one kind of ceramic selected from the groupconsisting of an oxide, nitride and carbide of iron, cobalt, nickel,titanium, barium, manganese, zinc, magnesium, aluminum, silicon, andcopper; or that the ceramic particles are bonded onto the surface of thecomposite magnetic particles to unify the ceramic particles and thecomposite magnetic particles; or that most of the ceramic particlesexist inside the crystalline grain and the grain boundary of themagnetic metal grains. It is preferable that the magnetic metal is asoft magnetic metal.

Further, the composite magnetic particles in accordance with the presentinvention are composite magnetic particles in which a magnetic metalparticle and a ceramic are unified by embedding and mixing finely innm-order the grains of a ceramic, such as metal oxide, inside a magneticmetal particle of soft magnetic ultra fine crystals. The high magneticpermeability obtained by finely crystallizing the soft magnetic metaland the high electric resistivity obtained by dispersing the ultra-fineceramic grains are attained at the same time. Therefore, a high magneticpermeability can be maintained and better absorbing characteristics arealso maintained even in the high frequency region.

Further, since the composite magnetic particle is formed byalternatively laminating a soft magnetic metal phase and a metal oxidephase, the width of the soft magnetic metal phase extends below the skindepth, and, accordingly, there is an effect equivalent to dispersingsoft magnetic metal powder having a thickness below the skin depth.Therefore, the eddy currents can be reduced, and electromagnetic wavescan be efficiently taken in. Further, by changing the mixing ratio andthe combination of the metal oxide phase and the ferromagneticultra-fine crystalline metallic phase, the parameters relating to thecharacteristic of electromagnetic wave absorption of complex specificmagnetic permeability and the complex specific dielectric constant canbe comparatively freely controlled, and, therefore, a bettercharacteristic of electromagnetic absorption can be obtained in a targetfrequency band.

In regard to the mixing ratio of the added ceramic particles, when thevolumetric mixing ratio of ceramics is below 20 volumetric % to that ofthe soft magnetic metal particles, the electric resistivity is notimproved sufficiently. Further, when the volumetric mixing ratio ofnon-magnetic ceramics is above 80 volumetric %, the magneticpermeability of the composite magnetic particles is decreased in anexcessively low level to deteriorate the characteristic ofelectromagnetic wave absorption. From these facts, it is preferable thatthe volumetric mixing ratio of ceramics is 30 to 60 volumetric %.

In accordance with the present invention, the magnetic metal powder andthe ceramic powder are unified by mixing them with each other in anultra-fine state through the mechanical alloying method. The method ofmanufacturing an electromagnetic wave absorber in accordance with thepresent invention is characterized by the fact that composite magneticparticles are formed, in which the magnetic metal grains and theceramic, preferably, above 10% in volume ratio, are unified. Further,the method of manufacturing an electromagnetic wave absorber inaccordance with the present invention constitutes what is called amechanical alloying method in which a composite powder, composed of amagnetic metal powder and a ceramic powder, and metallic balls orceramic balls are placed in a pot. The size of the ball is larger thanthe grain size of the metallic powder, and the amount of balls is largerthan the amount of the composite powder, preferably, in a ratio of 50 to100 balls to 1 of the composite powder in weight. The pot is thenrotated at a high speed, preferably 1500 to 3000 rpm to mix and unifythe magnetic metal powder and the ceramic powder in an ultra-fine stateby adding strong energy to the powders. By this method, the compositemagnetic particles are formed, in which the plurality of fine magneticmetal grains and the ceramic are unified.

That is, the method of manufacturing the electromagnetic absorber inaccordance with the present invention is characterized by the fact thatthe composite magnetic particles, in which more than 10% of theultra-fine magnetic metal grains and the ceramic particles aredispersed, are formed through method generally called an alloying methodin which the composite powder composed of the magnetic metal powder andthe ceramic powder is mixed and unified into an ultra-fine state. Sincethe composite magnetic particles have a high electric resistivity in thehigh frequency region due to the ultra-fine state, a high magneticcharacteristic can be obtained. Therefore, high electromagnetic waveabsorption characteristic can be obtained.

The electromagnetic wave absorber in accordance with the presentinvention is characterized by the fact that the composite magneticparticles described above, preferably, 20 to 70 weight % of thecomposite magnetic particles, are dispersed in a material having anelectric resistivity higher than the electric resistivity of thecomposite magnetic particles, particularly, a resin, an insulation paintor a ceramic sintered material. Therein, the reason why the compositemagnetic particles are dispersed in a material having an electricresistivity higher than that of the composite magnetic particles is thatthe electric resistivity of the composite magnetic particle itself isnot small enough to satisfy the requirements of an electromagnetic waveabsorber, and the freedom of designing the electromagnetic wave absorberis increased by changing the mixing ratio of the composite magneticparticles. It can be said from this viewpoint that the compositemagnetic material is better in a particle form than in a thin film form.

From the above, the electromagnetic wave absorber in accordance with thepresent invention, which is composed of composite magnetic particles,can be widely used in various application. For example, noise radiatingin a semiconductor element can be prevented by mixing theelectromagnetic wave absorber in a sealing resin of a resin sealing typesemiconductor package; or, electromagnetic waves generated in anelectronic circuit board itself can be absorbed by mixing theelectromagnetic wave absorber into an electronic circuit board made ofresin or an electronic circuit board made of ceramic or a metal oxide;or, an internal interference can be prevented by applying theelectromagnetic wave absorber together with an insulation paint onto aninner surface of an electronic casing made of a metal.

A composite member in accordance with the present invention ischaracterized by the fact that the composite member comprises compositemagnetic particles having a grain size smaller than 10 μm in which theplurality of magnetic metal grains with ceramic above 20% in volumeratio are unified; or the composite magnetic particles in which theplurality of fine magnetic metal particles and the ceramic are unifiedby enclosing the plurality of fine magnetic metal grains with theceramic; or it comprises composite magnetic particles in which themagnetic metal grains and the ceramic are unified by embeddingbar-shaped ceramics into the magnetic metal grains.

A composite member in accordance with the present invention ischaracterized by the fact that the composite member comprises any one ofor a combination of composite magnetic particles having a grain sizesmaller than 10 μm in which the magnetic metal grains and the ceramic,preferably above 10% in volume ratio, are unified; and compositemagnetic particles in which the plurality of fine magnetic metalparticles and the ceramic are unified by enclosing the plurality of finemagnetic metal grains with the ceramic; and composite magnetic particlesin which the magnetic metal grains and the ceramic are unified byembedding ceramics into the magnetic metal grains. The composite membercan be manufactured through a method similar to that described above.

The present invention relates to a composite member formed bycompounding composite magnetic particles, in which magnetic metal grainsand ceramics are unified, and a material having an electric resistivityhigher than that of the composite magnetic particles.

Further, the present invention relates to a composite member formed bycompounding composite magnetic particles, in which magnetic metal grainsand ceramics are unified, and at least one kind of a resin having anelectric resistivity higher than that of the composite magneticparticle, such as alumina and silica.

It is preferable that the ceramic is contained as 10 to 75 vol % in thecomposite magnetic particle and has a granular structure dispersed inthe magnetic metal grains. Further, in accordance with the presentinvention, it is preferable that the composite magnetic particles areformed by unifying fine crystals of a magnetic metal having an averagegrain size below 50 nm, preferably, below 20 nm, and ceramics of above10 volume %, preferably, 15 to 70 volume %, and that an average crystalgrain size of the composite magnetic particle is smaller than 50 nm.Further, the materials of the magnetic metal grains and the ceramic arethe same as described above.

The electromagnetic wave absorber is characterized by the fact that thesurface of the composite magnetic particles is coated with a materialhaving an electric resistivity higher than the electric resistivity ofthe composite magnetic particles; and that the composite magneticparticles have an aspect ratio larger than 2 and an oblate shape; andthat the composite magnetic particles are uniformly dispersed in thematerial having a high electric resistivity; and that the oblatecomposite magnetic particles are oriented in one direction in thematerial having a high electric resistivity; and that the materialhaving a high electric resistivity is a polymer material or a ceramicsintered material.

By forming the composite magnetic particle so as to have a structuresuch that the ceramic phase having a high electric resistivity enclosesthe ultra-fine magnetic metal crystals, as described above, the electricresistivity can be improved in the GHz region compared to the use ofsingle phase metal particles, and, in addition, the complex specificmagnetic permeability can be also improved.

Therein, when the crystal grain size of the magnetic metal composing thecomposite magnetic particles, the exchange interaction between metalcrystals is weakened to deteriorate the soft magnetic characteristic.Therefore, the magnetic permeability is decreased, and the electricresistivity is increased.

As a result, the crystal grain size of the magnetic metal composing thecomposite magnetic particles in accordance with the present invention ispreferably below 50 nm, particularly preferably below 20 nm.

Further, by controlling the volume ratio of the ceramics in thecomposite magnetic particles, the parameters relating to theelectromagnetic wave absorption characteristic of the complex specificmagnetic permeability and the complex specific dielectric constant canbe controlled. Therefore, a good electromagnetic wave absorptioncharacteristic in a target frequency band can be obtained. When thevolumetric mixing ratio of ceramic to the magnetic metal is below 10volume %, the complex specific magnetic permeability becomes highbecause the electric resistivity is not sufficiently increased, but thecomplex specific magnetic permeability is rapidly decreased in the GHzregion because of eddy current loss. Further, the imaginary part of thecomplex specific dielectric constant becomes too large to obtain asufficient electromagnetic wave absorption characteristic. Particularlyin the case where the ceramic phase is non-magnetism, when thevolumetric mixing ratio of the ceramic exceeds 70 volume %, the realparts of the complex specific magnetic permeability and the complexspecific dielectric constant of the composite magnetic particle aredecreased too low. Therefore, in order to obtain a sufficientelectromagnetic wave absorption characteristic, the electromagnetic waveabsorber needs to have a rather large thickness. For the above reason,it is preferable that the volumetric mixing ratio of the ceramic is 15to 70 volume % relative to the soft magnetic metal grains.

The electromagnetic wave absorber in accordance with the presentinvention is characterized by the fact that the composite magneticparticles of, preferably, 20 to 80 volume % are dispersed in a materalhaving an electric resistivity higher than the composite magneticparticles, particularly, a resin, an insulation paint or a ceramicsintered material.

In accordance with the present invention, by forming the compositemagnetic particle so as to have a structure such that the ceramic phasehaving a high electric resistivity encloses the fine magnetic metalcrystals, the electric resistivity can be improved in the GHz regioncompared to the commonly used single phase metal particle, and, inaddition, the complex specific magnetic permeability can be alsoimproved.

Further, by controlling the volume ratio of the ceramics in thecomposite magnetic particles, the parameters relating to theelectromagnetic wave absorption characteristic of the complex specificmagnetic permeability and the complex specific dielectric constant canbe controlled. Therefore, a good electromagnetic wave absorptioncharacteristic in a target frequency band can be obtained. When thevolumetric mixing ratio of the ceramic phase to the magnetic metal phaseis below 20 volume %, the electric resistivity is not sufficientlyincreased. Particularly, when the volumetric mixing ratio of the ceramicexceeds 70 volume %, the magnetic permeability of the composite magneticparticle is decreased too low. Therefore, the thickness of theelectromagnetic wave absorber can not be made thinner. For the abovereason, it is preferable that the volumetric mixing ratio of the ceramicis 20 to 70 volume % to the soft magnetic metal grains.

The reason why the composite magnetic particles are dispersed in thematerial having an electric resistivity higher than that of thecomposite magnetic particle is {circle around (1)} that the electricresistivity of the composite magnetic particle itself is notsufficiently high to perform as an electromagnetic wave absorber, and{circle around (2)} that the real part of the complex specificdielectric constant can be made large because a micro capacitor can befound using the composite magnetic particle in an electrode, and {circlearound (3)} that the frequency characteristics of the complex specificmagnetic permeability and the complex specific dielectric constant canbe controlled by controlling the particle shape and the dispersing formof the composite magnetic particles, and {circle around (4)} that thefrequency characteristics of the complex specific magnetic permeabilityand the complex specific dielectric constant can be controlled bycontrolling the volumetric mixing ratio of the composite magneticparticles relative to the insulation resin.

In accordance with the present invention, the three phase structure ofthe magnetic metal phase, the high electric resistivity ceramic phaseand the insulation material which is formed by unifying the compositemagnetic particles with the insulation material having an electricresistivity higher than that of the composite magnetic particle ispreferable compared to the two layer structure such as the compositebody of magnetic metal single phase particles and insulation resin orthe composite body of magnetic metal single phase particles and ceramic.

Therein, in order to further improve the electromagnetic wave absorptioncharacteristics, it is preferable that the composite magnetic particleis formed to have an oblate shape with an aspect ratio larger than 2 anda thickness less than the skin depth, and the oblate composite magneticparticles are orientated in the material having the high electricresistivity. That is, the electromagnetic wave absorptioncharacteristics can be further improved and thinning of theelectromagnetic wave absorber can be attained by suppressing of therapid decrease in the complex specific magnetic permeability due to eddycurrents, and by increasing of the magnetic permeability by decreasingthe effect of a demagnetizing field due to the particle shape andincreasing of the magnetic resonance frequency by the shape magneticanisotropy, and by improving the real part of the complex specificdielectric constant by increasing the area of the capacitor electrodes.

The methods of unifying the fine crystal grains of the magnetic metal(hereinafter, referred to as magnetic metal grains) and the ceramicapplicable to the present invention are as follows. That is, themechanical alloying method; and a method in which an alloy powder, whichis composed of a magnetic metal and an element having an affinity withoxygen, nitrogen and carbon higher than the magnetic metal and which hasa high content of any one of these gas elements, is fabricated throughthe atomizing method, and then the soft magnetic metal phase and theceramic phase are separately produced by performing heat treatment ofthe alloy powder; and a method in which an alloy powder, which iscomposed of a magnetic metal and an element having an affinity withoxygen, nitrogen and carbon higher than the magnetic metal and which hasa high content of any one of these gas elements, is fabricated throughthe atomizing method, and then heat treatment of the alloy powder isperformed in a gas atmosphere containing any one of oxygen, nitrogen andcarbon; and a method in which the soft magnetic metal phase and theceramic phase are separately produced; and a sol-gel method using metalalkoxide. The manufacturing method is not limited to the above-describedmethods, but a manufacturing method capable of finally obtaining thecomposite magnetic particles composed of the magnetic metal grain phaseand the high electric resistivity ceramic phase may be used as well.

In order to increase the electric resistivity of the composite magneticparticle itself, it is possible that a high electric resistivity film,such as an oxide film or a nitride film, is formed on the surface of thecomposite magnetic particle at the same time the composite magneticparticles are produced.

Further, it is possible to coat the surface of the composite magneticparticle with a material having a higher electric resistivity through amechanical unifying method, preferably through the mechano-fusion methodusing a kind of shearing type mill.

The composite magnetic particles are kneaded with an insulation polymermaterial of 30 to 80 volume %. Examples of the preferable insulationpolymer materials are polyester group resins; polyvinyl chloride groupresins; polyvinyl butylal resin; polyurethane resin; cellulose groupresins; copolymer of these resins; epoxy resin; phenol resin; amidegroup resins; imide group resins; nylon; acrylic resin; syntheticrubber; and so on. Epoxy resin is preferable. When the volumetricfilling ratio of the composite magnetic particles relative to the resinis above 50 vol %, the electric resistivity of the resin composite bodyis decreased due to contact between the composite magnetic particlesthemselves. Therefore, it is necessary to add, at a time, a couplingtreatment agent of silane group, alkylate group or titanate group, or amagnesium phosphate-borate insulation treatment agent.

As described above, by coating the surfaces of the composite magneticparticles with high electric resistivity material using the surfaceoxidation method, the mechanical unifying method or the chemical surfacetreatment method solely or in combination, the real parts of the complexspecific magnetic permeability and the complex specific dielectricconstant can be improved and the electromagnetic wave absorptivity canbe improved while keeping the electric resistivity at a constant valueeven if the mixing ratio of the composite magnetic particles relative tothe resin is increased.

The following applications of the electromagnetic wave absorber of thepresent invention are given as an example.

(1) In a semiconductor integrated device of the resin seal type, thecomposite magnetic particles are mixed in the sealing resin to suppressradiant noises in a semiconductor element level.

(2) In a printed wiring board, the paint comprising the electromagneticwave absorber of the present invention is directly applied or a filmwherein the paint is formed in a sheet-shape is attached onto a part ofor the whole of both of a surface having a wiring circuit formed thereonand a surface of the insulation board not having a wiring circuit so asto form an electromagnetic wave absorption layer. Thereby, theoccurrence of noise, such as a cross-talk, due to electromagnetic wavesgenerated from the printed wiring circuit board can be suppressed.Particularly, high density and high integration of a multi-layer wiringcircuit board can be attained with high reliability. The multi-layerwiring circuit board is constructed such that a first-layer wiring layeris formed as at least one side main surface of the semiconductor board,an insulation film is formed on the surface of the first-layer wiringlayer, a second-layer wiring layer electrically connected to thefirst-layer wiring layer through a via-hole is formed on the insulationfilm, and this process is repeated to form multi-layer wiring circuits.(3) A cap made of the composite magnetic particles and the materialhaving an electric resistivity higher than that of the compositemagnetic particles is mounted on a printed wiring board so as to enclosea semiconductor element operating as a noise source. Thereby, theelectromagnetic waves emitted from the semiconductor element can beefficiently absorbed and the electromagnetic wave internal interferencecan be suppressed.

The insulation paint containing the composite magnetic particles isapplied onto the inner surface of a metal electronic equipment casing,or an electronic equipment casing formed of the composite magneticparticles and a resin is used. Thereby, the electromagnetic waveinternal interference can be suppressed.

Further, the present invention is characterized by a semiconductordevice in which a semiconductor element mounted on a printed wiringboard is sealed with a resin containing an electromagnetic waveabsorber, wherein the resin in the side of the element is covered with aresin free from the electromagnetic wave absorber. The present inventionis also characterized by a printed wiring board comprising a wiringcircuit on an insulation board, and the circuit is covered with aninsulation layer, wherein layers comprising an electromagnetic waveabsorber are formed on a surface of the insulation board opposite to thesurface having the wiring circuit formed thereon and on the insulationlayer.

Further, the semiconductor device of the present invention isconstructed such that a semiconductor element mounted on a printedwiring board is covered with a metal cap of which an inner peripheralsurface is formed of an electromagnetic wave absorber; or asemiconductor element mounted on a printed wiring board is covered witha cap having an electromagnetic wave absorber; or a printed wiring boardand a semiconductor element mounted on the board are covered with acasing having an electromagnetic wave absorber. It is preferable thatthe material described above is used for the electromagnetic waveabsorber used in each of the semiconductor devices of the presentinvention.

In accordance with the present invention, in an optical sending orreceiving module comprising an electric-optical converter used for ahigh speed communication network, by covering an optical sending elementor an optical receiving element and the related circuit with anelectromagnetic wave absorber having composite magnetic particles andthe ceramic, or with an electromagnetic wave absorber in which thecomposite magnetic particles and a material having an electricresistivity higher than that of the composite magnetic particle areunified, the electromagnetic wave emitted outside the module and thenoise interference inside the module can be suppressed. Theelectromagnetic wave absorber used in accordance with the presentinvention is the same as described above.

According to the present invention, the electromagnetic wave absorbercomposed of the composite magnetic particles in which the magnetic metaland the non-magnetic or magnetic ceramics are unified in ultra-finelydispersed manner can obtain the remarkable effect of having an excellentelectromagnetic wave absorption characteristic compared to theelectromagnetic wave absorber made of the simply mixed powder.

Further, according to the present invention, the electromagnetic waveabsorber is formed by unifying the composite magnetic particles, each ofwhich is composed of fine crystal grains of the magnetic metal (themagnetic metal grains) and the ceramic phase, particularly, fine crystalgrains including at least one kind of non-magnetic or magnetic oxide,carbide and nitride, and the material having an electric resistivityhigher than that of the composite magnetic particles. Theelectromagnetic wave absorber has a good electromagnetic wave absorptioncharacteristic in the high frequency region, particularly, in the GHzregion, and can be formed as a thin electromagnetic wave absorber, andcan efficiently suppress electromagnetic wave interference inside theelectronic equipment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a microscopic photograph (TEM photograph) showing a crosssection of Fe—SiO₂ magnetic composite grains in accordance with thepresent invention.

FIG. 2 is a graph showing a measured result of the frequencycharacteristics of the magnetic permeability of magnetic compositegrains in accordance with the present invention and the magneticpermeability of a comparative mixed powder.

FIG. 3 is a graph showing a measured result of the frequencycharacteristics of the dielectric constant of the magnetic compositegrains in accordance with the present invention and the dielectricconstant of the comparative mixed powder.

FIG. 4 is a graph showing a measured result of the frequencycharacteristics of reflectivity of the magnetic composite grains inaccordance with the present invention and the reflectivity of thecomparative mixed powder.

FIG. 5 is a photograph of an image produced by a high resolutiontransmission electron microscope showing a cross section of a compositemagnetic particle in accordance with the present invention.

FIG. 6 is a graph showing the frequency characteristic of the complexspecific magnetic permeability of a composite magnetic particle in whichthe magnetic metal phase and the ceramic phase are unified in nano-meterlevel.

FIG. 7 is a graph showing the frequency characteristic of the complexspecific dielectric constant of the composite magnetic particle in whichthe magnetic metal phase and the ceramic phase are unified in nano-meterlevel.

FIG. 8(a)(1) is a graph showing the frequency characteristic of thereflection coefficient of the composite magnetic particles in which themagnetic metal phase and the ceramic phase are unified in nano-meterlevel in an electromagnetic wave absorber having a metal plate on oneside thereof, as shown in FIG. 8(a)(2).

FIG. 8(b)(1) is a graph showing the frequency characteristic of thereflection coefficient of the composite magnetic particles in which themagnetic phase and the ceramic phase are unified in nano-meter level inan electromagnetic wave absorber having no metal plate thereon, as shownin FIG. 8(b)(2).

FIG. 9 is a cross-sectional view showing an electromagnetic waveabsorber in which oblate composite magnetic particles are oriented in aresin.

FIG. 10 is a cross-sectional view showing a semiconductor integratedelement which is molded in a package with a sealing resin mixed with thecomposite magnetic particles.

FIG. 11 is a cross-sectional view showing a printed wiring board havingan electromagnetic wave absorption layer formed of the electromagneticwave absorber in accordance with the present invention.

FIGS. 12(a) and 12(b) are cross-sectional views showing anelectromagnetic wave absorption cap arranged on a printed wiring boardso as to enclose a semiconductor element of a noise source.

FIGS. 13(a) and 13(b) are cross-sectional views showing an electronicequipment casing formed of the electromagnetic wave absorber inaccordance with the present invention.

FIG. 14 is a cross-sectional view showing an optical sending modulewhich is completely sealed with a resin mixture containing the compositemagnetic particles, and in which the outside is further covered with ametal casing.

FIG. 15 is a cross-sectional view showing the optical sending module ofwhich the metal casing is removed.

FIG. 16 is a cross-sectional view showing an optical sending module oftwo-layer structure in which only the wiring portion is sealed with aninsulation resin not containing the composite magnetic particles, andthe outside of the insulation resin is sealed with a resin mixturecontaining the composite magnetic particles.

FIG. 17 is a schematic diagram showing a first form of an opticalsending and receiving module.

FIG. 18 is a schematic diagram showing the construction of a tollgateusing an electronic toll collection system (ETC) in which theelectromagnetic wave absorber in accordance with the present inventionis arranged in the ceiling surface of the gate roof and columns.

FIG. 19 is a diagram showing an electromagnetic wave absorber having amulti-layer structure in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(Embodiment 1)

A mixed powder consisting of Fe powder of 50 vol % having grain size of1 to 5 μm and SiO₂ powder of 50 vol % having an average grain size of0.3 μm, and balls made of SUS410 (diameter: 9.5 mm) with a weight ratioof the powders to the balls=1 to 80 were put into a pot made ofstainless steel, and the pot was filled with argon gas, and MA(mechanical alloying) treatment was performed thereon at a rotationspeed of 200 rpm for 100 hours. The composite magnetic particles afterthe MA was of indefinite shape, having complicated shapes, and theaverage particle size was several tens μm.

FIG. 1 is a photograph of the structure obtained from observation of thecomposite magnetic particle using a TEM. The crystal grain size of Fe ofthe black portion in the photograph is 10 nm, and the composite magneticparticle has a complicated shape, and Si oxide of the white portion isformed in a network-shape so as to enclose Fe grains having a grain sizebelow 100 nm. The fine Fe grains having a grain size below 20 nm wereindependently formed, and the complicated shaped Fe grains having agrain size larger than the grain size of the fine grains were formed bygathering the fine grains. Further, the Si oxide was dispersed in the Fecrystal grain boundaries, and the Fe grains and the Si oxide werealternatively formed in an oblate shape. Further, the Si oxide was alsoformed in a bar-shape, and Si oxide grains having a diameter below 0.05μm and a length of 0.2 to 0.5 μm were formed with a density of 10 to 20in number per 1 μm square.

Further, after MA, annealing of the composite magnetic particles wasperformed in a vacuum (the degree of vacuum: above 10⁻⁶ Torr) at atemperature of 500° C. for 1 hour. After that, the composite magneticparticles of 50% in volume ratio to epoxy resin were kneaded with theepoxy resin and press-formed into a tablet shape, and then the tabletswere cured by uniaxially pressing with 210 kgf at 180° C. After that,the cured tablets were finished in a toroidal shape of 7−0.05 mm outerdiameter, 3.04+0.06 mm inner diameter, 2 mm and 4 mm thickness.

When a complex specific dielectric constant and a complex specificmagnetic permeability of the sample were measured using a measurementsystem composed of a network analyzer (a product of HP: 8720C) and acoaxial waveguide, after calibrating so that the magnetic permeabilityand the dielectric constant of the free space might become 1, the samplewas inserted into the coaxial waveguide to measure two parameters S11and S21 using two ports, and then the complex specific dielectricconstant and the complex specific magnetic permeability were calculatedfrom the measured parameters.

Further, when a reflection characteristic was measured, aftercalibrating so that the reflection coefficient of the free space mightbecome 0, the sample was inserted into the coaxial waveguide to measurea parameter S11, and then the reflection coefficient of the sample wascalculated from the measured parameter. The range of measured frequencywas 50 MHz to 20 GHz.

In order to investigate the effect of the composite magnetic particlesin which the insulation metal oxide particles were dispersed in a softmagnetic metal particle, measurements of complex specific magneticpermeability, complex specific dielectric constant and frequencycharacteristics of reflection coefficient were performed using theFe—50vol % SiO₂ manufactured through the method of the present inventionand a sample which was manufactured by performing mechanical millingtreatment of Fe powder and SiO₂ powder separately under the samecondition as that of the MA treatment and then annealing the powders,and after that, simply mixing the two annealed powders using a V-mixerand then forming the composite structure with epoxy resin. The resultsof comparison are shown in FIG. 2 to FIG. 4.

It can be understood from FIG. 2 that both the real part and theimaginary part of the complex specific magnetic permeability for thecomposite magnetic particle sample in the high frequency region arehigher than those for the sample provided by simply mixing the Fe powderand the SiO₂ powder using the V-mixer.

It can be understood from FIG. 3 that both the real part and theimaginary part of the complex specific dielectric constant for thecomposite magnetic particle sample is slightly decreased due to thecomposite structure, and accordingly it is easy to adjust the impedancematching with the free space.

FIG. 4 shows the frequency characteristics of the reflectioncoefficients in a case of a sample thickness of 108 mm. The reflectioncoefficient for the composite magnetic particle sample is smaller, andthe central frequency (a frequency where the reflection coefficientbecomes minimum) for the composite magnetic particle sample exists inthe lower frequency side. Further, the frequency band width satisfyingthe reflection coefficient below −10 dB is wider in the compositemagnetic particle sample.

It can be understood from the above results that the electromagneticabsorption characteristic for the composite magnetic particle unified inthe soft magnetic metal powder and the insulation metal oxide in thenano-meter scale is improved compared with that for the sample obtainedby simply mixing the two kinds of powders.

(Embodiment 2)

A mixed powder consisting of Fe powder having a grain size of 1 to 5 μmand soft magnetic metal oxide powder of (Ni—Zn—Cu)Fe₂O₄ or (Mn—Zn)Fe₂O₄(50:50 in volume ratio) having an average grain size of 0.7 μm, andballs made of SUS410 (diameter: 9.5 mm) with a weight ratio of thepowders to the balls=1 to 80 were put into a pot made of stainlesssteel, and the pot was filled with argon gas, and MA (mechanicalalloying) treatment was performed at a rotation speed of 200 rpm for 100hours. The composite magnetic particles after the MA was of indefiniteshape, and the average particle size was several tens μm. Further, theresult of observing the composite magnetic particle using a TEM wassimilar to that of Embodiment 1. The crystal grain size of Fe was about10 nm, and oxides including components of the soft magnetic metal oxidewere finely dispersed in a network shape in the crystal grain boundary.Annealing of the composite magnetic particles was performed in a vacuum(the degree of vacuum: above 10⁻⁶ Torr) at temperature of 500° C. for 1hour. The composite magnetic particle showed a structure similar to thatof Embodiment 1.

In order to investigate the effect of the composite magnetic particles,measurements of various characteristics were performed using thecomposite magnetic particles according to the present invention and asample which was manufactured by performing mechanical milling treatmentof Fe powder and soft magnetic metal oxide powder separately under thesame condition as that of the MA treatment and then annealing thepowders, and after that, simply mixing the two annealed powders using aV-mixer and then forming the composite structure with epoxy resin. Asresults of comparison, an effect similar to that of Embodiment 1 wasobtained.

(Embodiment 3)

A powder obtained by mixing Fe powder having a grain size of 1 to 5 μmand Si powder having an average grain size of 1.0 μm of 50:50 in volumeratio, and the same balls made of SUS410 as described above with aweight ratio of the powders to the balls=1 to 80 were put into a potmade of stainless steel together, and the pot was filled with oxygen gas(Ar:O₂=4:1), and mechanical alloying (MA) treatment was performed at arotation speed of 200 rpm for 100 hours. The composite powder after theMA was of indefinite shape, and the average particle size was 5.0 μm.Further, as a result of observing the composite magnetic particle usinga TEM, the crystal grain size of Fe was about 10 nm, and oxidesincluding components of Si oxide were finely dispersed in a networkshape in the crystal grain boundary. Further, as a result of an X-raydiffraction analysis, it was checked that there were Fe oxides (Fe₂O₃,Fe₃O₄). Similarly to the method described above, various kinds ofcharacteristics of the composite magnetic particles mixed with epoxyresin were measured. As a result, a structure and characteristicssimilar to those of the composite magnetic particles manufacturedthrough the method of Embodiment 1 were obtained.

(Embodiment 4)

The particle surfaces of the composite magnetic particles obtained fromEmbodiments 1 to 3 were coated with a non-magnetic or magnetic oxidehaving a high electric resistivity. The coating method used was asurface oxidation method or a mechanical composition method.

By setting the atmospheric condition at annealing in the manufacturingprocess of the composite magnetic particles to the atmosphere or anoxygen atmosphere as the surface oxidation method, it was checked froman X-ray diffraction analysis that oxides such as Fe₃O₄ were produced.

On the other hand, a mechano-fusion method using a kind of shearing typemill was employed as the mechanical composition method. In detail, thecomposite magnetic particles (average particle size: 10 μm) were used asthe host particles, and SiO₂ (average particle size: 0.016 μm) or(Ni—Zn—Cu)Fe₂O₄ (average particle size: 0.5 μm) were used as the guestparticles. The host particles and the guest particles were mixed in thevolume ratio of 2:3, and then put into the mechano-fusion apparatus. Theconditions of mechano-fusion were in a vacuum, rotating speed: 1000 rpm,and treatment time: 3 hours. As a result, it was checked from SEMobservation that the surfaces of the composite magnetic particles werecoated with relatively compact oxide film of about 1.0 μm thicknessformed of the guest particles.

(Embodiment 5)

A mixed powder consisting of Fe powder of 70 vol % having grain size of1 to 5 μm and SiO₂ powder of 30 vol% having an average grain size of 0.3μm, and balls made of stainless steel were put into a pot made ofstainless steel, and the pot was filled with argon gas, and mechanicalalloying treatment was performed. The composite magnetic particles afterthe mechanical alloying was of indefinite shape, and the averageparticle size was several tens μm. After that, annealing of thecomposite magnetic particles was performed in a vacuum (the degree ofvacuum: above 10⁻⁶ Torr) at a temperature of 500° C. for 1 hour.

The method of unifying the fine crystal grains of the magnetic metal(hereinafter, referred to as magnetic metal grains) and the ceramicgrains is not limited to the mechanical alloying method described above.For example, the following methods are applicable. That is, a method inwhich an alloy powder, which is composed of a magnetic metal and anelement having an affinity with oxygen, nitrogen and carbon higher thanthe magnetic metal and has a high content of any one of these gaselements, is fabricated through the atomizing method, and then the softmagnetic metal phase and the ceramic phase are separately produced byperforming heat treatment of the alloy powder; and a method in which analloy powder, which is composed of a magnetic metal and an elementhaving an affinity with oxygen, nitrogen and carbon higher than themagnetic metal and has a high content of any one of these gas elements,is fabricated through the atomizing method, and then heat treatment ofthe alloy powder is performed in a gas atmosphere containing any one ofoxygen, nitrogen and carbon; and a method in which the soft magneticmetal phase and the ceramic phase are separately produced; and a sol-gelmethod using metal alkoxide. The manufacturing method is not limited tothe above-described methods, but a manufacturing method capable offinally obtaining the composite magnetic particles composed of themagnetic metal grain phase and the high electric resistivity ceramicphase may be used.

In order to increase the electric resistivity of the composite magneticparticle itself, it is possible that a high electric resistivity film,such as an oxide film or a nitride film, is formed on the surface of thecomposite magnetic particle at the same time producing the compositemagnetic particles.

Further, it is possible to coat the surface of the composite magneticparticle with a material having a higher electric resistivity through amechanical unifying method, preferably through the mechano-fusion methodusing a kind of shearing type mill. In detail, the composite magneticparticles (average particle size: 10 μm) were used as the hostparticles, and SiO₂ (average particle size: 0.016 μm) or (Ni—Zn—Cu)Fe₂O₄(average particle size: 0.5 μm) were used as the guest particles. Thehost particles and the guest particles were mixed in the volume ratio of2:3, and then put into the mechano-fusion apparatus (preferably in avacuum, rotating speed: 1000 rpm, and treatment time: 3 hours). As aresult, it was checked from SEM observation that the surfaces of thecomposite magnetic particles were coated with a relatively compact oxidefilm of about 1.0 μm thickness formed of the guest particles.

FIG. 5 is a TEM photograph of the composite magnetic particle annealedunder a vacuum after the mechanical alloying treatment. The blackportion in the photograph was fine crystal grains of Fe, and the crystalgrain size was 10 to 20 nm. Amorphous Si oxide existed so as to enclosethe fine crystal grains of Fe.

Then, after drying and crushing treatment, the composite magneticparticles were press-formed into a tablet shape under room temperature.Further, the tablets were cured by uniaxially pressing with 210 kgf at180° C. Also, as other methods of manufacturing the resin compositebody, there are the injection molding method, the transfer mold methodand so on. When a sheet-shaped resin composite body is manufactured, thedoctor blade method, the spin coat method, and the calendar roll methodare applicable.

These resin composite bodies were finished in a toroidal shape of 7−0.05mm outer diameter, 3.04+0.06 mm inner diameter, 0.5 to 2mm thickness bymachining and grinding. Next, in regard to the characteristic evaluationmethod, when a complex specific dielectric constant and a complexspecific magnetic permeability of the sample were measured using ameasurement system composed of a network analyzer (a product of HP:8720C) and a coaxial waveguide, after calibrating so that the magneticpermeability and the dielectric constant of the free space might become1, the sample was inserted into the coaxial waveguide to measure twoparameters S11 and S21 using two ports, and then the complex specificdielectric constant and the complex specific magnetic permeability werecalculated using the Nicolson-Ross, Weir method from the measuredparameters.

Further, when a reflection characteristic was measured, aftercalibrating so that the reflection coefficient of the free space mightbecome 0, the sample was inserted into the coaxial waveguide to measurea parameter S11, and then the reflection coefficient of the sample wascalculated from the measured parameter. The range of measured frequencywas 0.1 to 18 GHz.

In order to compare the characteristics of the composite magneticparticles with those of single phase Fe particles, a sample wasmanufactured by separately performing mechanical milling treatment of Fepowder having a grain size of 1 to 5 μm and SiO₂ powder having anaverage grain size of 0.3 μm under the same condition as that of themechanical alloying treatment, putting the Fe powder and the SiO₂ powderhaving a volume ratio of 70:30 together, and sufficiently mixing using aV-mixer, and then forming the mixed powder annealed in the samecondition as described above into the composite structure with epoxyresin through the same method as described above. The complex specificmagnetic permeability, the complex specific dielectric constant and thefrequency characteristics of reflection coefficient of the sample weremeasured.

FIG. 6 to FIG. 8 show comparison of the complex specific magneticpermeability, the complex specific dielectric constant and the frequencycharacteristics of reflection coefficient between the composite magneticparticles and the single phase Fe particles. It can be understood fromFIG. 6 that both of the real part and the imaginary part of the complexspecific magnetic permeability in the high frequency region for thecomposite magnetic particle sample are higher than those for the simplemixed powder of the Fe powder and the SiO₂ powder. It can be understoodfrom FIG. 7 that the real part of the complex specific dielectricconstant for the composite magnetic particles is larger than that of thesimple mixed powder, and the imaginary part for the composite magneticparticles is also slightly increased. FIG. 8(a) shows the frequencycharacteristic of reflection coefficient in a case where there is ametal plate on the one side of the electromagnetic wave absorber, and itis seen that the reflection coefficient for the composite magneticparticle is smaller. FIG. 8(b) shows the measured results of an amountof electromagnetic wave absorption of the electromagnetic wave absorberitself, and it is seen that the amount of electromagnetic waveabsorption for the composite magnetic particle is larger.

It can be understood from the above results that the electromagneticabsorption characteristic can be improved by unifying the soft magneticmetal grain phase and the high electric resistivity ceramic phase in thenano-meter scale.

(Embodiment 6)

In Embodiment 5, in cases where an alloy containing Ni, Co instead of Feor containing at least one ferromagnetic metal among these metals, forexample, parmalloy of Fe—Ni group, sendust of Fe—Al—Si group, Fe—Nialloy group, Fe—Cr alloy group, Fe—Cr—Al alloy group were used, and incases where alumina (Al₂O₃), Mn—Zn group ferrite, Ni—Zn group ferrite ofspinel group as a magnetic oxide, in addition, plannar type hexagonalferrite, magneto-planbite type ferrite were used instead of SiO₂, thesame effect could be obtained.

(Embodiment 7)

In order to make the shape of the composite magnetic particles aftermechanical alloying treatment in Embodiment 5 or 6 oblate, oblatecomposite magnetic particles having an aspect ratio above 2 wereobtained by putting the composite magnetic particles into a crusher,such as a planetary ball mill (or an attriter), together with an organicsolvent, such as ethanol, to perform wet treatment. After heattreatment, the oblate composite magnetic particles were mixed with aliquid resin to produce a paste state, then formed in a sheet-shapethrough the doctor-blade method in which shear force is applied to thecomposite magnetic particles, and then press-formed using a hot press.As the result of observing a cross section of the sheet using a SEM, theoblate composite magnetic particles were orientated, as shown in FIG. 9.

A composite compound of the oblate composite magnetic particles and aresin was manufactured in advance, and then injected to a metal moldusing an injection molding machine. As a result of observing a crosssection of the molded piece using a SEM, the oblate composite magneticparticles were highly orientated, as shown in FIG. 9. In the case wherethe oblate composite magnetic particles are highly oriented in theresin, it was observed that the real parts of the complex specificmagnetic permeability and the complex specific dielectric constant wereimproved compared to those in Embodiments 5 and 6, and theelectromagnetic wave absorptivity was largely improved.

(Embodiment 8)

FIG. 10 is a cross-sectional view showing a semiconductor integratedcircuit device which is sealed with a sealing resin mixed with thecomposite magnetic particles described in Embodiments 1 to 7. As shownin FIG. 10, by molding packages with a sealing resin mixed with thecomposite magnetic particles in the manufacture of microprocessors orLSIs, electromagnetic waves generated from ICs and inner leads composingthe semiconductor integrated circuit are absorbed to suppress theinternal interference. By covering the semiconductor element side of thesealing resin mixed with the composite magnetic particles with a resinwhich is composite magnetic particle free, it is possible to preventelectric contact with the lead. Electric connection between the ICs andthe externals is performed by solder balls 7 through the printed wiringboard 9. The leads 8 are made of any one of Au, Cu or Al wire.

(Embodiment 9)

FIG. 11 is a cross-sectional view showing a printed wiring board havingan electromagnetic wave absorption layer formed of the electromagneticwave absorber described with reference to Embodiments 1 to 7. In aprinted wiring board having a wiring circuit 13 in an insulation board9, a paint comprising the electromagnetic wave absorber composed of thecomposite magnetic particles and a material having an electricresistivity higher than that of the composite magnetic particles isdirectly applied, or a film in which the paint is formed into asheet-shape is attached onto a part or the whole of an insulation layer10 on the surface of the insulation board 9 having a wiring circuit 13formed thereon and the opposite surface of the insulation board 9 nothaving the wiring circuit to form an electromagnetic wave absorptionlayer. Thereby, occurrence of noises, such as by a cross-talkphenomenon, due to electromagnetic waves generated from the printedwiring circuit board. can be suppressed. Further, by arranging aconductive layer outside each of the electromagnetic wave absorptionlayers, the electromagnetic wave absorptivity can be improved, and theshielding effect on electromagnetic waves from the outside can beimproved.

(Embodiment 10)

FIGS. 12(a) and 12(b) are cross-sectional views showing anelectromagnetic wave absorption cap arranged on a printed wiring boardso as to enclose a semiconductor element of a noise source. Theelectromagnetic wave absorption cap in accordance with the presentinvention is arranged on a printed wiring board so as to enclosesemiconductor elements appearing as noise sources, such as amicroprocessor, a system LSI etc. FIG. 12(a) shows a case where theelectromagnetic wave absorption layer in accordance with the presentinvention is arranged on the inner surface of the metal cap, andelectromagnetic waves from the outside can be shielded andelectromagnetic waves emitted from the inside can be absorbed. FIG.12(b) shows a case where a cap molded by injection molding of theelectromagnetic wave absorber in accordance with the present inventionis used. By mounting the cap, the electromagnetic waves emitted from thesemiconductor element can be efficiently absorbed to suppress theinternal interference.

(Embodiment 11)

FIGS. 13(a) and 13(b) are cross-sectional views showing an electronicequipment casing formed of the electromagnetic wave absorber inaccordance with the present invention. FIG. 13(a) shows a case where theelectromagnetic wave absorption layer in accordance with the presentinvention is applied onto the inner surface of a metal casing forelectronic equipment, or the electromagnetic wave absorption layerformed through injection molding is arranged on the inner surface. FIG.13(b) shows a case where the electronic equipment casing is molded byinjection molding of the electromagnetic wave absorber in accordancewith the present invention. By adding the function of absorbingelectromagnetic waves to the electronic equipment casing as describedabove, electromagnetic wave interference inside the electronic devicecan be suppressed.

(Embodiment 12)

FIG. 14 is a view showing the construction of an optical sending modulein accordance with the present invention. The optical sending module 21comprises an optical fiber 25, an optical guide path 29, an LD 26, asending circuit 27, a circuit board 28 etc. The sending circuit 27 iscomposed of an LD driver for driving the LD 26 of a laser diode, a laseroutput control part, a flip-flop circuit and so on. Actually, there area lead frame, wiring and so on, but these elements are not shown in thefigure. In the present embodiment, the optical sending module isperfectly sealed by putting it into a mold, by pouring the resin mixturecontaining the composite magnetic particles described Embodiments 1 to 7into the mold, and by curing the resin mixture. Further, the outside ofthe molded optical sending module is covered with a metal casing 30. Bydoing so, the elements and the board can be protected from water andgas, and at the same time electromagnetic waves can be absorbed andshielded so as to suppress noise interference inside the sending module,and emission of electromagnetic waves outside the module can becompletely prevented.

The metal casing 30 is not always necessary. Therefore, as shown in FIG.15, the module may be only sealed with the resin mixture. This structureis inferior to the above case covered with the metal casing inabsorption and shielding effects of electromagnetic waves, but has anadvantage of low cost.

Further, short circuiting between the wires can be prevented by coatingthe surfaces of the composite magnetic particles with insulation. As amethod of coating insulation, there are a method in which a film havingan electric resistivity, such as an oxide film or a nitride film, isformed on the surface of the composite magnetic particle by heattreatment in an atmosphere; a chemical film forming method using acoupling treatment agent of silane group, alkylate group or titanategroup, or a magnesium phosphate-borate insulation treatment agent; and amechanical film forming method in which the surface of the compositemagnetic particle is coated with a material having a higher electricresistivity through the mechano-fusion method using a kind of shearingtype mill.

Further, a more reliable method of preventing short circuiting betweenthe wires is a two-layer structure in which only the wiring portions aresealed with an insulating resin not containing the composite magneticparticles, and then the resin mixture containing the composite magneticparticles is sealed thereon, as shown in FIG. 16.

The particle size of the composite magnetic particles is preferablybelow 40 μm when taking the fluidity of the resin mixture intoconsideration, though the size depends on the composition of thecomposite magnetic particle. The shape of the composite magneticparticles may be spherical or oblate. The filling amount of thecomposite magnetic particles relative to the resin is preferably below60 vol % from the viewpoint of securing the fluidity of the resinmixture. The usable resins, in addition to epoxy group resin commonlyused as sealing resin of electronic equipment, are polyester groupresins; polyvinyl chloride group resins; polyvinyl butylal resin;polyurethane resin; cellulose group resins; copolymer of these resins;epoxy resin; phenol resin; amide group resins; imide group resins;nylon; acrylic resin; synthetic rubber; and so on.

Although the present embodiment has been described with reference to theLD 26 and the sending circuit 27, an optical receiving module may besimilarly constructed by replacing these by a PD and receiving circuit.

(Embodiment 13)

FIG. 17 is a schematic diagram showing a first form of an opticalsending and receiving module. The optical sending and receiving module23 comprises the functions of both the optical sending module and theoptical receiving module described above. The optical sending portioncomprises an optical fiber 25, an optical guide path 29, an LD 26, asending circuit 27, a circuit board 28 and so on. The sending circuitcomprises an LD driver for driving a laser, a laser output controlportion, a flip-flop circuit and so on. The optical receiving portioncomprises an optical fiber 25, an optical guide path 29, a PD 35, areceiving circuit 36, a circuit board 28 and so on. The receivingcircuit comprises a PRE IC having a pre-amplifying function, a CDR LSIcomposed of a clock extraction portion and an equivalent amplifier, anSAW of a narrow band filter, an APD bias control circuit and so on.Actually, there are a lead frame, wiring and so on, but these elementsare not shown in the figure.

In the sending and receiving module integrating the sending module andthe receiving module, the internal noise interference due to noiseproduced during sending and receiving between the optical sendingportion and the optical receiving portion particularly becomes aproblem, as described above.

In the present embodiment, the arrangement of the electromagnetic waveabsorber can be constructed similarly to that of Embodiment 12, as shownin FIG. 14 to FIG. 16.

In a conventional optical sending and receiving module, noiseinterference is prevented by arranging a shield plate made of a metalbetween the sending portion and the receiving portion, or by enclosingeach of the modules into a package made of a metal to form a separatesending module and a separate receiving module. However, such a modulehas problems in that the whole module becomes large in size and heavy inweight, and, in addition, the cost can not be lowered because of use ofthe high cost metal package. By employing the structure of the presentinvention, the noise interference inside the module can be prevented,and the module can be made small in size, light in weigh and low incost.

Further, according to the present embodiment, it is possible to providean optical sending module, an optical receiving module or an opticalsending and receiving module having both an optical sending portion andan optical receiving portion, which are capable of being used in a highspeed communication network, and which can suppress internal noiseinterference and noise emission to the outside, and can be made small insize, light in weight, high in processing speed and high in sensitivity.

(Embodiment 14)

FIG. 18 is a diagram showing the construction of a tollgate in which anelectronic toll collection system (hereinafter, referred to as ETC) isemployed. The ETC is capable of sending and receiving informationbetween a road side communication unit and an in-car unit mounted on avehicle passing through the tollgate.

As shown in FIG. 18, electromagnetic waves at a frequency of 5.8 GHz areused among an entrance portion antenna 40, an exit portion antenna 41and the in-car unit 41 to exchange information necessary for paying andreceiving a toll. The spread of the electromagnetic waves sent from theexit portion antenna 41 (direct wave 46) becomes wider due to anelectromagnetic wave multi-reflection phenomenon with the road surface43 and the ceiling 44 of the gate roof 43 or columns 45. Thereby, asshown in FIG. 18, it can be expected that an erroneous operation will becaused due to electromagnetic wave disturbance, such as a problem ofinterference by a vehicle in the adjacent lane and a problem ofdistinction between vehicles, such that the electromagnetic wavestransmitted from the exit portion antenna 41 (direct wave 46) that aresent to the in-car unit of the vehicle A48 will, at the same time,produce a reflected wave 47 reflected by the roar surface 43 are sent toan in-car unit 42 of the following vehicle B48. Therefore, the aboveproblem can be solved by arranging the electromagnetic wave absorberscontaining the composite magnetic particles in the ceiling surface ofthe gate roof 44 and the columns to absorb the reflected wave.

A conventional electromagnetic wave absorber for ETC is of an integratedtype, and the thickness is as thick as several tens cm. Therefore, it isdifficult to attach it onto a portion having a complex shape.Accordingly, development of an electromagnetic wave absorber in the formof a paint or a soft and thin material is required. The electromagneticwave absorber 49 is made of a resin mixture containing the compositemagnetic particles and can be formed into a paint or a soft sheetdepending on selection of the resin. Further, the composite magneticparticles are excellent particularly in the electromagnetic wavecharacteristics in the high frequency region above 5 GHz compared to theconventional soft magnetic metal particles. Therefore, these problemscan be solved by the electromagnetic wave absorber in accordance withthe present invention.

The electromagnetic wave absorber 49 using a resin mixture containingcomposite magnetic particles may be formed in a single layer structure.However, in order to improve the oblique incident characteristic, it ismore effective if it is formed in a multi-layer structure in which theimpedance of the electromagnetic wave absorber to the incident wave 50is gradually decreased from the wave incident surface toward the side ofthe metal layer of a perfect reflector. In detail, the complex specificmagnetic permeability and the complex specific dielectric constant aregradually decreased from the wave incident surface toward the side ofthe metal layer 51. In order to do so, the filling amount of thecomposite magnetic particles of the same composition to the resin isvaried, or the composition of the composite magnetic particles in theresin is varied. Therein, the metal layer is not necessary when theattached surface is made of a metal. In FIG. 19, the electromagneticwave absorber 49 is composed of three layers.

The particle size of the composite magnetic particle is preferably below40 μm when taking the fluidity of the resin mixture into consideration,though the size depends on the composition of the composite magneticparticle. The shape of the composite magnetic particle may be sphericalor oblate, but the invention is not particularly so limited. The fillingamount of the composite magnetic particles relative to the resin foreach layer is preferably 60 vol % at maximum from the viewpoint ofsecuring the fluidity of the resin mixture. The usable resin may be anyinsulation polymer, and the resins described in Embodiment 12 arepreferable.

1. An electromagnetic wave absorber comprising composite magneticparticles having a grain size smaller than 10 μm in which magnetic metalgrains and ceramic are unified, wherein said composite magneticparticles are substantially oblate composite magnetic particles and aresubstantially oriented in one direction in said material having a highelectric resistivity.
 2. An electromagnetic wave absorber comprisingcomposite magnetic particles in which a plurality of fine magnetic metalgrains and ceramic are unified by enclosing said plurality of finemagnetic metal grains with said ceramic, wherein said composite magneticparticles are substantially oblate composite magnetic particles and aresubstantially oriented in one direction in said material having a highelectric resistivity.
 3. An electromagnetic wave absorber comprisingcomposite magnetic particles in which magnetic metal grains and aplurality of ceramic grains are unified by embedding the ceramic grainsinto the magnetic metal grains, wherein said composite magneticparticles are substantially oblate composite magnetic particles and aresubstantially oriented in one direction in said material having a highelectric resistivity.
 4. An electromagnetic wave absorber formed bycompounding together both composite magnetic particles, in whichmagnetic metal grains and ceramics are unified, and a material having anelectric resistivity higher than an electric resistivity of thecomposite magnetic particles, wherein said composite magnetic particlesare substantially oblate composite magnetic particles and aresubstantially oriented in one direction in said material having a highelectric resistivity.
 5. An electromagnetic wave absorber according toany one of claims 1 and 2 to 4, wherein said magnetic metal is at leastone kind of metal or alloy selected from the group consisting of iron,cobalt and nickel, and said ceramic is at least one kind of ceramicselected from the group consisting of oxide, nitride and carbide ofiron, aluminum, silicon, titanium, barium, manganese, zinc, magnesium,cobalt and nickel.
 6. An electromagnetic wave absorber according to anyone of claims 1 and 2 to 4, wherein a volume ratio of said ceramic tothe composite magnetic particle is 10 to 75%, and said ceramic isembedded in said magnetic metal grains.
 7. An electromagnetic waveabsorber according to any one of claims 1 and 2 to 4, wherein an averagecrystal grain size of said composite magnetic particle is smaller than50 nm.
 8. An electromagnetic wave absorber according to any one ofclaims 1 and 2 to 4, wherein the surface of said composite magneticparticle is coated with a material having an electric resistivity higherthan an electric resistivity of said composite magnetic particle.
 9. Anelectromagnetic wave absorber according to any one of claims 1 and 2 to4, wherein said composite magnetic particle has an aspect ratio largerthan 2, and has an oblate shape.
 10. An electromagnetic wave absorberaccording to any one of claims 1 and 2 to 4, wherein said compositemagnetic particles are uniformly dispersed in said material having thehigh electric resistivity.